This invention relates to 2-substituted 4-[3-(4H-benz[b]pyran-4-ylidene)prop-1-enyl]benz[b]pyrylium, 4-[5-(4H-benz[b]pyran-4-ylidene)penta-1,3-dienyl]benz[b]pyrylium and 4-[7-(4H-benz[b]pyran-4-ylidene)hepta-1,3,5-trienyl]benz[b]-pyrylium dyes, hereinafter for brevity referred to simply as "bisbenzpyrylium" dyes. This invention also relates to processes for the preparation of these dyes and processes for their use as visible or near infra-red absorbers.
The term "near infra-red" is used herein to mean electromagnetic radiation having a wavelength of about 700 to about 1200 nm.
Visible and near infra-red absorbers, that is to say compounds which absorb visible and/or near infra-red electromagnetic radiation, are known to be useful in a number of applications. For example, such absorbers are useful as filters used in photography and for protecting sensitive materials from visible and infra-red radiation, and in inks used to print characters intended to be read using visible or near infra-red light--for example, in universal product code labels intended to be scanned with an infra-red laser. Such absorbers are sometimes useful as sensitizers to render photographic film sensitive to red or near infra-red radiation, and some of them can act as charge transfer materials and are thus applied to the photosensitive surfaces used in xerography and in other types of electrophotography.
A wide variety of compounds have been used as visible and infra-red absorbers, including benzthiazoles, metal dithiolenes and phthalocyanines. Benzpyrylium and benzthiopyrylium dyes have also been used for this purpose.
For example, Canadian Patent No. 1,247,915 discloses a bis(4-benzpyrylium) pentamethine dye used as an infra-red sensitizer. The nuclei each carry a 2-phenyl substituent and a 7-acetyl group.
U.S. Pat. Nos. 4,173,473 and 4,233,443, disclose bis(4-pyrylium) and bis(4-benzpyrylium) trimethine compounds for use in radiation sensitive compositions; in these compounds, the trimethine chain is aryl-substituted.
U.S. Pat. No. 4,714,667, issued Dec. 22, 1987, describes an optical information recording medium comprising a recording layer on a substrate and an optional protective layer; the recording layer comprises a hexa-, penta-, tetra- or trimethine compound wherein three of the carbon atoms of the polymethine chain form part of a ring, and each end of the polymethine chain bears any one of a variety of heterocyclic groups, including benzpyrylium groups. The two specific compounds disclosed containing 4-benzpyrylium groups (compounds 11 and 27) both have 2-phenyl substituents.
U.S. Pat. No. 4,857,431, issued Aug. 15, 1989, describes a photoconductive composition comprising an inorganic photoconductive material, a sensitizing dye and a resin binder. The dye comprises a penta- or heptamethine compound wherein each end of the polymethine chain bears any one of a variety of heterocyclic groups, including benzpyrylium groups. These heterocyclic groups may bear alkyl and other substituents.
U.S. Pat. No. 4,283,475, issued August 11, 1981 describes 2,6-di-tert-butyl-4-[5-(2,6-di-tert-butyl-4H-thiopyran-4-ylidene)penta-1,3 -dienyl]-thiopyrylium salts, which are colorless, transparent, have an adsorption in the far infrared to near infrared region and are capable of imparting high sensitivity to photoconductive substance; a process for producing the salts; and a photoconductive composition containing the salts.
Japanese Patent Application No. 103,604/82 (Publication No. 220,143/83, published Dec. 21, 1983), discloses a broad class of bis-heterocyclic pentamethine dyes in which the central three carbon atoms of the pentamethine chain form part of a squarylium or croconylium ring. The heterocyclic nuclei can be pyrylium, thiopyrylium, selenopyrylium, benzpyrylium, benzthiopyrylium, benzselenopyrylium, naphthopyrylium, naphthothiopyrylium or naphthoselenopyrylium nuclei, which can be substituted with alkyl, alkoxy, aryl or styryl groups.
Japanese Patent Application No. 60-8730 (Publication No. 167,681/86, published Jul. 29, 1986), discloses bis(4-benz[b]thiopyrylium pentamethine dyes in which the central three carbon atoms of the pentamethine chain for part of a squarylium ring. The dyes are intended for use as infra-red absorbers.
West German Offenlegungsschrift No. 38 34 960 discloses a bis(2-t-butyl-4-benzpyrylium) monomethine dye for use in a photopolymerizable composition.
Visible and infra-red absorbers have the capacity to absorb infra-red radiant energy and convert it to heat, thereby heating any medium in which the infra-red absorber is present, and several applications of such absorbers depend upon such generation of heat. Thus, visible and infra-red absorbers can be used to induce chemical or physical changes in the medium containing them, and such chemical or physical changes can be induced with an appropriate wavelength of light. For example, a layer of visible or infra-red absorber spread over a supporting plate can undergo visually perceptible physical distortions upon illumination by an infra-red laser, so that the layer acts as an optical recording medium, as in a compact disc. For example, U.S. Pat. No. 4,508,811, issued Apr. 2, 1985, describes an optical recording element in which the recording layer comprises a bis(2,6-dialkyl) pyrylium or thiopyrylium squarylium salt.
U.S. Pat. No. 4,555,472 describes an optical recording member comprising a bis(2,6-diaryl-4-pyrylium) trimethine compound, in which the central carbon atom of the trimethine chain may bear an alkyl or aryl substituent.
U.S. Defensive Publication No. 889,023 describes bis(benzpyrylium) pentamethine and heptamethine dyes in which the benzpyrylium nuclei bear 2-phenyl, 7-acetoxy-2-(2,4-dimethoxyphenyl) and 7-acetoxy-2-phenyl substituents. These compounds are used as sensitizers in electrophotographic elements.
Such chemical and physical changes can also be used in thermal imaging; the highly localized heating produced by the absorber can be used to create a high resolution image. For example, U.S. Pat. No. 4,720,449, issued Jan. 19, 1988, describes a thermal imaging method which comprises heating imagewise a di- or triarylmethane compound possessing within its di- or triarylmethane structure an aryl group substituted in the ortho position to the meso carbon atom with a moiety ring-closed on the meso carbon atom directly through a nitrogen atom, which nitrogen atom is also bound to a group with a masked acyl substituent that undergoes fragmentation upon heating to liberate the acyl group for effecting intramolecular acylation of the nitrogen atom to form a new group in the ortho position, whereby the di- or triarylmethane compound is rendered colored in an imagewise pattern corresponding to the imagewise heating.
U.S. Pat. No. 4,602,263 and U.S. Pat. No. 4,826,976 both describe thermal imaging systems for optical recording and particularly for forming color images. This thermal imaging method relies upon the irreversible unimolecular fragmentation of one or more thermally unstable carbamate moieties of an organic compound to effect a visually discernible color shift from colorless to colored, from colored to colorless or from one color to another.
International Patent Application No. PCT/US87/03249 (Publication No. WO 88/04237), published Jun. 16, 1988, describes a different type of thermal imaging system using an element comprising a support formed of a material transparent to radiation of a specific wavelength and having an imaging surface layer heat activatable at an elevated temperature, and a layer of porous or particulate imaging material uniformly coated on the imaging surface layer and exhibiting a cohesive strength which is greater than the adhesive strength between the imaging material and the imaging surface layer. When this element is illuminated with radiation of the specific wavelength, at least one of the materials used in the two layers absorbs this radiation, thus heat activating the imaging surface layer and locking substantially the entire layer of the imaging material to the support when the imaging surface layer cools. After exposure of the element, a peeling force is applied to the imaging material so that in the unexposed areas of the element, the imaging material will peel from the support; however, in the exposed areas, the locking of the imaging material causes this material to be retained upon the support.
In many thermal imaging systems in which a leuco dye is transformed into a colored compound by heat, the leuco dye often does not have sufficient absorption at a convenient wavelength to permit it to convert sufficient radiation to heat to effect the color change. For example, in the forementioned U.S. Pat. Nos. 4,602,263 and 4,826,976, many of the leuco dyes absorb in the ultraviolet. In such thermal imaging systems, it is normally preferred to use a laser as the radiation source, and at present ultraviolet lasers are not well-suited to imaging processes, and such processes are preferably carried out using an infra-red laser. Accordingly, it is preferred to include with the leuco dye an infra-red absorber for converting infra-red radiation into heat, which is transferred to the leuco dye to effect the color change.
Similarly, in the thermal imaging system described in the aforementioned International Patent Application No. PCT/US87/03249, an infra-red absorber may be provided in a layer adjacent the imaging surface layer to assist in converting infra-red radiation into heat.
The requirements for visible and infra-red absorbers for use in thermal imaging systems are stringent. Since the sensitivity and the resolution of the image produced are often affected by the thickness of the layers in the heat-sensitive element (the sensitivity of the system is inversely related to the mass of material required to be heated, and thus inversely related to the thickness of the relevant layers), it is necessary to provide a high degree of absorption of radiation within a thin layer, sometimes of the order of 1 .mu.m. To produce this degree of absorption, it is necessary that the absorber used have a high extinction coefficient, of the order of at least about 100,000, and a low molecular weight. In addition, in many cases it is desirable that the absorber manifest its maximum absorption within the range of about 700-1200 nm. so that it can conveniently be used with existing near infra-red lasers. (In the present state of technology, solid state diode lasers emitting at about 760 to 1000 nm. provide the highest output per unit cost. YAG lasers emitting at about 1000-1200 nm. are also useful in thermal imaging processes.)
One major problem with many prior art benzpyrylium dyes is that they have low solubility in most plastics and in the semi-polar solvents (for example, methyl ethyl ketone and methylene chloride) from which they need to be deposited to form imaging media such as those used in the aforementioned International Patent Application No. PCT/US87/03249. Thus, it is difficult to dissolve or disperse the absorber in a plastic without forming aggregates and without adversely affecting other properties of the plastic.
Accordingly, there is still a need for development of improved visible and near infra-red absorbers.
The aforementioned co-pending application Ser. No. 07/616,639, discloses that 4-[[3-[(benz[b]-4H-pyran-4-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1- ylidene]methyl]benz[b]pyrylium hydroxide inner salt dyes (i.e., bisbenzpyrylium pentamethine dyes in which the central three carbon atoms of the pentamethine chain form part of a squarylium nucleus) in which the benzpyrylium nuclei carry 2-phenyl substituents have very low solubilities in most plastics and in semi-polar solvents (for example, methyl ethyl ketone and methylene chloride) from which they need to be deposited to form many types of imaging media. The application states that the solubility of bisbenzpyrylium pentamethine dyes can be substantially improved by providing on at least one of the benzpyrylium nuclei, a 2-substituent in which a non-aromatic carbon atom is bonded directly to the benzpyrylium nucleus; preferably, at least one of the benzpyrylium nuclei should also bear a bulky 7-substituent which extends substantially out of the plane of the benzpyrylium nucleus.
The present inventors have found that bis(2-phenyl-benzpyrylium) dyes in which the two benzpyrylium nuclei are linked by a straight polymethine chain do not share the very low solubilities in plastics and semi-polar solvents of bis(2-phenyl-benzpyrylium) dyes in which the two benzpyrylium nuclei are linked by a polymethine chain containing a squarylium ring; the straight-chain dyes can readily be incorporated into polymeric media such as those in most imaging systems. However, the present inventors have found, although these straight-chain dyes can be incorporated into polymeric media, the spectral characteristics of the dyes in such media are undesirable in a thermal imaging system.
The infra-red spectrum of a typical straight-chain dye of this type shows only a single major peak in solution (hereinafter referred to as "the primary peak"). However, when the straight-chain dye is dispersed in a plastic, a second infra-red absorption (hereinafter referred to as "the secondary peak") occurs at a substantially shorter wavelength, and this shorter wavelength absorption is typically stronger than the longer wavelength absorption. The development of the secondary peak is accompanied by a dramatic reduction in absorption at the wavelength of the primary peak, and neither peak displays as high an absorption as does the dye in solution. The resultant diminution in maximum absorption is highly disadvantageous in imaging systems (for example, that described in the aforementioned International Patent Application No. PCT/US87/03249) in which it is necessary to absorb a substantial fraction of incident radiation in a very thin layer, of the order of 1 .mu.m.
It has now been found that the tendency for bis(2-phenylbenzpyrylium) straight chain dyes to develop strong absorptions at shorter wavelengths (and thus to absorb over a wide range of wavelengths) when dispersed in plastic films can be substantially reduced by providing, on at least one of the benzpyrylium nuclei, a 2-substituent in which a non-aromatic carbon atom is bonded directly to the benzpyrylium nucleus.